fix: mock agent process, React 18/19 types, release version refs

- Add hidden `mock-agent-process` CLI subcommand implementing a stdio
  JSON-RPC echo agent (ported from examples/mock-acp-agent)
- Update write_mock_agent_process_launcher() to exec the new subcommand
  instead of exiting with error
- Update sdks/react to support React 18 and 19 peer dependencies
- Update @types/react to v19 across workspace (pnpm override + inspector)
- Fix RefObject<T | null> compatibility for React 19 useRef() signatures
- Add version reference replacement logic to release update_version.ts
  covering all docs, examples, and code files listed in CLAUDE.md
- Add missing files to CLAUDE.md Install Version References list
  (architecture.mdx, boxlite, modal, computesdk docs and examples)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sdks/react/package.json

@@ -24,7 +24,7 @@
     "typecheck": "tsc --noEmit"
   },
   "peerDependencies": {
-    "react": "^18.3.1",
+    "react": "^18.3.1 || ^19.0.0",
     "sandbox-agent": "^0.2.2"
   },
   "dependencies": {
@@ -32,8 +32,8 @@
     "ghostty-web": "^0.4.0"
   },
   "devDependencies": {
-    "@types/react": "^18.3.3",
-    "react": "^18.3.1",
+    "@types/react": "^19.1.12",
+    "react": "^19.1.1",
     "sandbox-agent": "workspace:*",
     "tsup": "^8.0.0",
     "typescript": "^5.7.0"

sdks/react/src/AgentConversation.tsx

@@ -18,7 +18,7 @@ export interface AgentConversationProps {
   emptyState?: ReactNode;
   transcriptClassName?: string;
   transcriptClassNames?: Partial<AgentTranscriptClassNames>;
-  scrollRef?: RefObject<HTMLDivElement>;
+  scrollRef?: RefObject<HTMLDivElement | null>;
   composerClassName?: string;
   composerClassNames?: Partial<ChatComposerClassNames>;
   transcriptProps?: Omit<AgentTranscriptProps, "entries" | "className" | "classNames" | "scrollRef">;

sdks/react/src/AgentTranscript.tsx

@@ -98,8 +98,8 @@ export interface AgentTranscriptProps {
   entries: TranscriptEntry[];
   className?: string;
   classNames?: Partial<AgentTranscriptClassNames>;
-  endRef?: RefObject<HTMLDivElement>;
-  scrollRef?: RefObject<HTMLDivElement>;
+  endRef?: RefObject<HTMLDivElement | null>;
+  scrollRef?: RefObject<HTMLDivElement | null>;
   scrollToEntryId?: string | null;
   sessionError?: string | null;
   eventError?: string | null;

sdks/react/src/useTranscriptVirtualizer.ts

@@ -4,7 +4,7 @@ import type { RefObject } from "react";
 import { useEffect, useRef } from "react";
 import { useVirtualizer } from "@tanstack/react-virtual";
 
-export function useTranscriptVirtualizer<T>(items: T[], scrollElementRef?: RefObject<HTMLDivElement>, onAtBottomChange?: (atBottom: boolean) => void) {
+export function useTranscriptVirtualizer<T>(items: T[], scrollElementRef?: RefObject<HTMLDivElement | null>, onAtBottomChange?: (atBottom: boolean) => void) {
   const isFollowingRef = useRef(true);
 
   const virtualizer = useVirtualizer({